Because we've failed to terraform much closer, less inhospitable environments, and the risks for getting it wrong are very high.
And by 'we've failed to terraform', did you mean 'we haven't tried yet'?
California's Central Valley project is an example, and China has some that are even larger.
Plus, there's no life that we know of on Venus. A bacterial population capable of living, evolving, and growing exponentially in that environment would be a huge change.
Worst case scenario is we'd lose some valuable scientific information about Venus in its natural state.
It seems like a remote risk to me, but not something to shrug off. Particularly since the usefulness of introducing life to Venus is itself quite remote.
Why not? It'd have to have different chemistry, sure, but even on Earth life exists in some pretty extreme temperature and environment ranges.
Life that could operate at 500 C is probably not water based at all, which would be very novel and certainly interesting to science.
To some weird soul out there on the Internet, this sounds like a challenge.